Electrically powered systems rely on a variety of switches for controlling their operation. These switches may be solid state devices such as SCRs or triacs, or may be electromechanical relays. (The term "switch" will be used hereafter to refer to any device having a pair of power terminals whose conductive state is controlled by an electrical signal on a control terminal of the device.) In either case it is important for safe operation of some systems that electrical power applied to them can be reliably controlled. In particular, many systems require near absolute certainty that power can be removed from them when desired. Combustion systems having fuel valves for controlling flow of pressurized fuel are one classic example of this situation.
Where solid state devices are involved, their failure mode is typically an open circuit which of course removes power from the system. A relay on the other hand, is notorious for failing with its switch contacts closed, so that removing power from the actuator coil does not remove power from the powered system. This condition of the contacts is referred to as welding. It is even possible that solid state switches can fail in a conducting mode, the solid state equivalent of relay contact welding. Because of their low switch resistance and the preferences of preexisting safety codes, relays are still usually used to switch power to the fuel valves in burner systems, so safe operation requires that relay contact welding not result in continued power flow to the controlled component of the system.
One expedient for increasing the reliability of disconnecting power from load for such switching systems is to use redundant switches, with two pairs of switch contacts in series. Thus if one pair of contacts weld, the other pair continues to safely provide switching for the powered system. One problem that arises with this arrangement however, is that once one contact pair welds, redundancy has been lost but the system continues to operate normally. The system is thus at risk of failure through welding of both contact pairs. Depending on how the switch control operates and the individual switch characteristics, it is possible that both switches will weld in near succession, say if both contact pairs have experienced approximately the same number of load switching operations. For systems where switch failure by welding creates an unsafe condition, the possibility of this type of failure should be avoided.